The invention relates to a process for the catalytic removal of polycyclic aromatic nitro, nitroso and/or amino compounds from an oxygen-containing exhaust gas of a combustion system, in particular a diesel engine.
It is known from the article xe2x80x9cNeue Gefahr im Dieselabgasxe2x80x9d [New danger in diesel exhaust gas] by Jens Simon, VDI-Nachrichten, Nov. 7, 1997, that compounds from the class of the nitrogenous, polycyclic aromatic hydrocarbons (nitro-PAHs) are contained particularly in the exhaust gas from a diesel engine. It is assumed that these nitro-PAHs are formed as a result of a reaction of the polycyclic aromatic hydrocarbons (PAHs) which accumulate on emitted soot particles as byproducts of the combustion of diesel with the nitrogen oxides contained in the exhaust gas. In particular, the presence of ozone appears to play an important role in this reaction. The higher the ozone level in the ambient air, the greater the level of nitro-PAHs in the exhaust gas from a diesel engine.
In the context of the present document, the term nitro-PAHs is to be understood as meaning polycyclic aromatic nitro and nitroso compounds, as well as polycyclic aromatic amino compounds.
The group of PAHs contained in the exhaust gas from a diesel engine includes, for example, benzpyrene, anthracene and naphthalene.
Various representatives of the group of the nitro-PAHs, in particular 3-nitrobenzanthrone and dinitropyrene, are highly toxic. For example, 3-nitrobenzanthrone is thought to be highly mutagenic and carcinogenic. Other representatives of the nitro-PAHs can also have a high toxicity.
Particularly in the case of a diesel engine for driving a passenger automobile or truck, not inconsiderable levels of nitro-PAHs are present whenever ozone is present close to the ground. In recent years, an increase in atmospheric pollution has led to increased formation of ozone in particular in the summer months.
It is accordingly an object of the invention to provide a process for catalytic removal of polycyclic aromatic nitro, nitroso and/or amino compounds (nitro-PAHs) from the exhaust gas of a combustion system, in particular a diesel engine, that overcomes the disadvantages and inefficiencies of prior art processes and devices of this general type, and allows the levels of nitro-PAHs contained in the exhaust gas to be reduced as simply and effectively as possible.
With the foregoing and other objects in view, there is provided, according to the invention, a process for the catalytic removal of polycyclic aromatic nitro, nitroso and/or amino compounds from the exhaust gas of a combustion system, in particular a diesel engine, which comprises the steps of transporting a gas stream containing a polycyclic aromatic nitrogen compound into a reactor volume containing a catalyst body, and contacting the gas stream with a catalyst body including an active mass of titanium dioxide at a temperature in the range of 150xc2x0 C. to 600xc2x0 C., whereby the polycyclic aromatic nitrogen compound is oxidized to nitrogen oxides, carbon dioxide, and water. Sufficient oxygen to accomplish this oxidation is present in the exhaust gas but can be supplemented with external supply where needed.
Initial tests have shown that, by bringing an exhaust gas which contains nitro-PAHs into contact with the surface of a catalytic converter which contains titanium dioxide, the nitro-PAH compounds, in the presence of oxygen, are very successfully oxidized to form nitrogen oxides, carbon dioxide and water. The catalytic converter containing titanium dioxide has hitherto been known as a so-called deNOx catalytic converter for reducing the levels of nitrogen oxides in the exhaust gas of a combustion system using the selective catalytic reduction process in the presence of oxygen and a reducing agent.
The catalytically active material advantageously contains 80 to 95% by weight titanium dioxide, 2-10% by weight tungsten trioxide and/or molybdenum trioxide, and 0.05 to 5% by weight vanadium pentoxide.
In order to oxidize the nitro-PAHs, a certain oxygen content is required in the exhaust gas. Particularly in the case of a diesel engine, the exhaust gas inherently has a relatively high residual oxygen content, so that by bringing the exhaust gas into contact with the catalytic converter, the levels of nitro-PAHs contained therein are effectively reduced. However, it also possible for oxygen to be separately admixed to the exhaust gas.
The catalytic converter can be constructed as a honeycomb body or as a plate-type catalytic converter. However, the catalytic converter can also be used as bulk material or in the form of pellets. Inorganic fibers and/or silicates can be added to the catalytically active material as filler and support matrix.
In an advantageous configuration of the invention, a nitrogen-containing reducing agent is added to the exhaust gas before it is brought into contact with the catalytic converter, which reducing agent, at the catalytic converter, in the presence of oxygen reduces nitrogen oxides contained in the exhaust gas. In this way, first the formation of the nitro-PAHs from PAHs and nitrogen oxides contained in the exhaust gas is suppressed. Secondly, in this way the catalytic converter is utilized as a deNOx catalytic converter for the removal of nitrogen oxides using the SCR process. Accordingly, the use of a reducing agent has a double benefit. Also, nitro-PAHs are converted at the catalytic converter, together with oxygen and the nitrogen-containing reducing agent, to form nonhazardous compounds. This is an additional reaction pathway in addition to the oxidation of the nitro-PAHs with oxygen to form carbon dioxide and water.
An example of a suitable nitrogen-containing reducing agent is ammonia or a compound which releases ammonia. Advantageously, urea is added as a compound which releases ammonia and from which ammonia is formed in the exhaust gas at a suitable temperature within the effective range by pyrolysis and/or hydrolysis in the water-containing exhaust gas.
The invention offers the advantage that, because of the use of a catalytic converter, the formation of nitro-PAHs from PAHs is prevented and, at the same time, the nitro-PAHs are broken down to form nonhazardous compounds. In principle, the invention can be used to reduce the levels of the highly dangerous nitro-PAHs in an exhaust gas from a combustion system if this exhaust gas contains oxygen. Otherwise, oxygen is supplied to the exhaust gas separately.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for the catalytic removal of polycyclic aromatic nitro, nitroso and/or amino compounds, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.